None of the vaccines in trials today elicit antibody (Abs) that neutralize a broad range of primary isolates of the type-1 human immunodeficiency virus (HIV-1). Three human Abs that neutralize a broad range of HIV-1 primary isolates have been cloned: MAbs bl2 and 2G12 and 2F5. Passive immunization with these MAbs produces sterilizing protection against IV and mucosal challenge of macaques with pathogenic SHIV strains. This supports the hypothesis that an active vaccine that stimulates the production of Abs similar or identical to MAbs bl2, 2F5 and 2G12 should provide effective prophylaxis against HIV- 1 infection. We propose to develop and optimize vaccine-lead peptides that would induce the production of neutralizing Abs having properties (and perhaps structures) similar to those of MAbs 2F5 and 2G12. We have already identified unique peptide ligands for both MAbs, and now propose to optimize them using a combination of phage display technology and synthetic peptides. The optimized peptides will be tested as immunogens in conjugate vaccines. With previous R21 funding, we developed B2.1, a homodimeric peptide that binds specifically and selectively to MAb bl2 (it is now in advanced stages of optimization), and have prepared conjugates and begun testing it as a vaccine. We have shown so far that the B2.1 peptide produces strong anti-peptide Ab titers when used as a conjugate vaccine in mice, and perhaps, weak cross-reactivity to gpl20 (this awaits further confirmatory testing). B2.1 also appears to identify bl2-like reactivities in the sera of some HIV-1 infected people. We plan similar work for the 2G12 and 2F5 leads; they are only in the very early stages of development. By means of functional and structural studies, we will probe the basis of the reactivity of the MAbs with their corresponding peptides in order to engineer high-affinity peptide ligands. We will test those peptides for their ability to elicit HIV-1-cross-reaetive Abs following two immunization strategies in mice, XenoMouse animals and macaques; and will then test the immune sera for the presence of HIV-1-binding Abs and for neutralizing activity. We will also assess the peptides for their ability to predict whether a HIV-1-infected person has made Abs that are similar to MAbs 2G12 and 2F5. We hypothesize that neutralizing Abs are produced throughout most of the HIV- 1 infection, albeit at very low levels, and thus, that "the right" vaccine should elicit these Abs in most people.